The present invention relates to a screw tightening apparatus that tightens a screw by motor driving.
Screw tightening apparatuses include one in which a motor serving as a driving source and a bit engaging with a recess of a screw are offset in their radial direction (for example, refer to Japanese Patent Laid-Open No. 2003-25164). In many cases, such a so-called offset-type screw tightening apparatus is used in, for example, a case in which a plurality of screw tightening apparatuses are disposed so as to be adjacent to one another, to collectively performing the tightening of a plurality of screws disposed with a pitch narrower than a pitch between the motors.
Further, in such an offset-type screw tightening apparatus, generally, a motor output is transmitted to an output shaft connected to a bit via a train of gears.
The output shaft is pressed down by a spring force in order to prevent the engagement between the bit and the recess from being canceled in screw tightening. Therefore, it is necessary for the output shaft to simultaneously perform two axial motions of rotation and axial sliding.
Conventionally, as a configuration enabling an output shaft to perform two axial motions of rotation and sliding, the following configuration is general. A hole is formed in the center of a gear (hollow gear) receiving a driving force from a motor, and an output shaft is made to pass through the hole. A key axially extending is provided to an inner circumference of the hollow gear, and this key is engaged with a key groove formed on the output shaft. With this configuration, the hollow gear and the output shaft are integrally rotatable due to spline coupling, and the output shaft is slidable along the hollow gear. Japanese Unexamined Utility Model Application Publication No. 6-53037 discloses a screw tightening apparatus having the similar configuration.
Meanwhile, in accordance with miniaturization of products such as precision apparatuses in recent years, screws used for the products are miniaturized, which results in a narrower pitch among screws. In order to tighten such fine screws disposed at an extremely narrow pitch, it is necessary to make diameters of a bit and an output shaft to which the bit is connected as well small.
However, as a diameter of the output shaft becomes small, it is difficult to precisely form a key groove on the output shaft. Even if a key groove can be precisely formed therein, because a width thereof is made extremely narrow, transmission of a sufficient torque from the hollow gear may be impossible.
Further, many products such as precision apparatuses into which screws are tightened include parts vulnerable to static electricity. Therefore, it is necessary to prevent static electricity generated on the bit and having harmful effects on the products from flowing into the products.
The output shaft is supported rotatably by ball bearings fixed to a main body of the screw tightening apparatus. However, some grease is provided among metal parts constituting the ball bearing, which reduces electrical conductivity therebetween.
Further, since some grease is applied between the hollow gear and the output shaft in order to make relative sliding thereof smooth in many cases, the electrical conductivity from the output shaft to the hollow gear as well is reduced. As described above, conventionally, it has been difficult to secure a route through which the static electricity generated on the bit is effectively flowed out.
Japanese Patent Laid-Open No. 2001-293666 discloses a configuration designed to eliminate static electricity from a screw tightening apparatus. Japanese Patent Laid-Open No. 2001-293666 discloses a configuration in which a conductive brush along which an outer circumference of a rotating output shaft is slidable is attached to a bearing case and a motor block which constitute a main body of the apparatus. However, in the screw tightening apparatus disclosed in Japanese Patent Laid-Open No. 2001-293666, the conductive brush attached to the main body of the apparatus touches the outer circumference of the output shaft which does not axially move with respect to the main body. Therefore, if this configuration is directly applied to a screw tightening apparatus whose output shaft axially moves, inconvenience such as an axial deformation of the conductive brush may be caused.